With the development of mobile electronic equipment, transportation, renewable-energy sectors, there is a strong demand for improved rechargeable battery systems, also known as secondary battery systems, with e.g. increased higher energy density. Compared to other secondary battery systems, systems based on lithium-ion batteries (LiBs) have many advantages in terms of high energy and power densities, long cycle life, low self-discharge, high operating voltage, wide temperature window, and no “memory effect”. Due to its long cycle life, abundant material supply and relatively low cost, graphite is the state-of-the-art active material used in the negative electrode, i.e. anode, of such LIBs. However, the graphite-based anodes show a low energy density (only 372 mAh/g) but also safety issues caused by lithium deposition under overcharge conditions. Therefore, much attention is paid to develop alternative active materials for such anodes that have enhanced safety, high specific capacity and also long cycle life.
Silicon (Si) is a (semi)metal of special interest because of its potentially largest theoretical capacity (around 3600 mAh/g for a Li15Si4 alloy). However, the implementation of Si-based anodes has been hindered by rapid capacity fading upon charge/discharge cycling. Without being bound by any theory, the capacity loss was believed to be mainly due to an increased expansion/contraction of the active material, i.e. crystalline Si, during the insertion/extraction of Li-ions (Li+), which leads to a strong mechanical stress of the Si crystallites and may result in the loss of electrical contact. Upon cycling, a rapid loss of reversible capacity was observed leading to poor battery performance.
Many strategies have been proposed to improve the cyclability of Si-based anode materials, such as (i) employing nano-technology to reduce the size of the Si-based particles, (ii) alloying Si with other elements and (iii) coating/mixing Si with carbon-based materials. One of the most appealing strategies proved to be the use of nano or sub-micron sized silicon based particles (such particles are also referred to as grains), to avoid cracking thereof during expansion and contraction upon cycling. However, the smaller the size of the particles, the higher their surface area; and a drawback thereof is that when such particles are contacted with an electrolyte which chemically interacts with the particles, the decomposition of said electrolyte is increased due to the larger available surface for unwanted reactions to occur.
An alternative material to pure Si is silicon oxide such as SiO. The so-called “Silicon monoxide” SiO, if it exists, would be the only compound of silicon in which silicon is bivalent. In recent years, experimental evidence taken via various methods has confirmed that silicon(II) oxide does not exist as a distinct phase but as a nano-scale mixture of crystalline Si and amorphous SiO2. It was also shown that such mixing occurs over a scale of 3 to 4 nm. Therefore, as proved by Schnurre et al. in Thermodynamics and phase stability in the Si—O system, J. Non-Cryst. Solids 2004, 336, 1-24, “amorphous SiOx is not a classical homogeneous single phase, yet because of this small domain size it is also not a classical heterogeneous two-phase mixture”. Therefore, amorphous SiOx with various amounts (x) of oxygen therein is characterized by many with the help of a random-mixture (RM) model, stating that over certain domains, silicon is bonded to only silicon or only oxygen and hence corresponding to an intimate, two-phase mixture of Si and SiO2. This is confirmed by 29Si MAS-NMR spectra of SiOx showing two different resonances whose chemical shift values are close to those of elemental state Si and SiO2, suggesting that the RM model is the most appropriate description for SiOx microstructure.
SiOx is potentially a parent material for Si-based anode materials owing to the irreversible generation of Li2O/Li4SiO4 and Si during the first lithiation (discharge) process. The Si particles formed in-situ during the first lithiation process are nano-sized and dispersed uniformly in a matrix containing a Li2O phase and a Li4SiO4 phase which are essentially simultaneously formed during said lithiation. Such matrix is an electrochemically inactive material which may have the ability to prevent the electrochemically active Si cluster from aggregating, and may thus improve the cycling stability of Si-based materials.
The commercially available SiOx is usually used as a powder that may be prepared by (1) using a method disclosed by US 2010/009261 A1 involving heating a mixture containing silicon and silicon dioxide in an inert gas atmosphere or in vacuum at a high temperature to generate SiO gas, and feeding oxygen gas to the SiO gas to form a gas mixture with various amounts x (usually x≧1) of oxygen, and depositing the gas mixture on a surface of a cooled substrate; (2) using a method according to US 2007/0254102 A1 involving mixing and depositing a gas mixture of SiO and Si gases on a substrate, the starting material to generate SiO gas being a mixture of a silicon oxide powder or a silicon dioxide powder with a metal silicon powder, where the x value is usually less than 1. However, such preparation methods of SiOx by simultaneously generating silicon and silicon oxide vapors and combining thereof with themselves and or with oxygen streams, necessitate a high working temperature (more than 2000° C.) due to the low vapor pressure of silicon and silicon oxide, which in turn may result in high cost and low yields.
As mentioned hereinabove, coating/mixing Si with carbon-based materials is also a strategy to obtain active materials that may provide LiBs using thereof with increased performance. Preparation methods of composites comprising SiOx/graphite, SiOx/carbon, and SiOx/graphite/carbon, may involve ball milling commercial SiOx powder with graphite to form SiOx/graphite composites. Other preparation methods of such composites may involve the formation of disordered carbon on the surface of SiOx particles by CVD, sol-gel, hydrothermal methods, etc., followed by a heat treatment. Such composite materials and their method of preparation are disclosed for example in US 2012/0115033; US 2005/0233213 and US 2006/0068287.
However, despite of all the latest advancements in the art of active materials suitable for utilization in the negative electrodes of LiBs, there is still a need for yet better materials that have the ability to further increase the performance of the batteries. In particular, for most applications batteries having increased capacity and reduced irreversibility are desirable. In an attempt to reach advantageous performances, the present inventors observed that the nature of the materials used in the manufacturing of the electrodes is of crucial importance. In particular they noticed that the nature of the SiOx is one of the most important parameters.